The present invention relates to the production and use of polyester polyols synthesized from at least one carboxylic acid anhydride and diethylene glycol, wherein the formation of 1,4-dioxane from diethylene glycol is substantially suppressed by a special reaction procedure.
Polyester polyols are an important constituent of many foamed and unfoamed polyurethane systems. Polyester polyols, as used for forming polyurethanes, have a large predominance of hydroxyl end groups available for further reaction with isocyanate groups. The molecular weight of polyester polyols is typically in the range 200-5000 dalton. They are mainly produced by the polycondensation of polycarboxylic acids, especially dicarboxylic acids, with polyols, especially diols, whereby carboxyl and hydroxyl groups are reacted to form ester groups under dehydrating conditions. An alternative possibility is to use polycarboxylic acid anhydrides, e.g. phthalic anhydride.
Dehydrating conditions can be achieved e.g. by the application of a vacuum, by the use of an inert gas stream to blow out the water of reaction, or by azeotropic purging with an entraining agent (Houben-Weyl, Methoden der organischen Chemie, volume 14/2, Makromolekulare Stoffe, Thieme Verlag Stuttgart, ed. E. Müller, pp 1-47, 1963).
Those skilled in the art are aware that when the aromatic acid phthalic acid, normally used in the form of phthalic anhydride, is esterified with diethylene glycol, 1,4-dioxane is formed as an unwanted by-product. During production in industrial plants, the dioxane formed is discharged together with the water of reaction and subsequently has to be degraded, e.g. in waste treatment plants, or concentrated and then incinerated. This additional process step increases the costs of polyester polyol production.
The 1,4-dioxane formed as a by-product also has the effect of reducing the yield of the desired product, since, as described, part of the diethylene glycol used is removed from the reaction mixture in the form of 1,4-dioxane instead of being incorporated into the polyester produced. The formation of 1,4-dioxane therefore constitutes serious economic disadvantage.
Furthermore, the amount of 1,4-dioxane that is allowed to be produced by a production plant can be limited by specified concessions. In these cases, a limit on the amount of dioxane thus leads indirectly to a limit on the production capacity of a polyester polyol production plant.